1. Field of the Invention
The present application generally relates to image processing and, more particularly, to an image processing apparatus, method, and storage medium, and a technology for performing a halftoning process on a line-containing image with high reproducibility.
2. Description of the Related Art
There are electrophotographic printers and inkjet printers that have been known as image output apparatuses that form images on recording media. In those image output apparatuses, the number of tone levels that can be output is often smaller than the number of tone levels in input digital image data. Therefore, a representation method involving area coverage modulation is used as a method of expressing shadings in an output image. Area coverage modulation is a method of expressing shadings by varying the proportion of the colorant applied area on a recording medium. Methods involving area coverage modulation are roughly classified into AM (amplitude modulation) screening and FM (frequency modulation) screening.
In AM screening, halftone dots are formed by spatially gathering and arranging minute dots, and shadings are expressed by varying the sizes of the halftone dots. In FM screening, on the other hand, minute dots are spatially scattered at random, and shadings are expressed by varying the density of the minute dots. In an image subjected to such screening, discontinuities might appear in fine lines. Fine line discontinuities are phenomena that occur when halftone dots for screening interfere with fine lines. In view of this, Japanese Patent Application Laid-Open No. 2007-300206 discloses a method of performing correction on a pre-screen-processing image to be input to a screen processing unit, using a gamma correction LUT specially designed for linear regions. By this method, the tonal values in the linear regions included in an image are corrected to be darker, so that fine line discontinuities due to screen processing can be corrected.
By the method disclosed in Japanese Patent Application Laid-Open No. 2007-300206, the tonal values in the linear regions in a pre-screen-processing image are corrected. As a result, the width of each line in an image that is output from an image output apparatus might appear greater than a desired width.